Mechanical timepiece movements equipped with a sprung balance and a Swiss lever escapement have been known for a long time. This escapement comprises a pallet-lever provided with a fork and a guard pin, an impulse pin integral with the balance and cooperating with the fork to provide the balance with impulses for maintaining its oscillation. Next, the timepiece movement further comprises two banking pins or solid bankings for limiting the rotation of the pallet-lever in both directions. These pins define two locking positions for the pallet-lever between which it oscillates.
During each vibration of its oscillation, the pallet-lever passes through various phases: A locking phase, an unlocking phase, an impulse phase and a safety phase. During the locking phase or period, the pallet-lever rests against a banking pin, the escape wheel is immobile and the impulse pin describes an ascending and then descending supplementary arc. The unlocking phase concerns the unlocking of an escape wheel tooth resting on a locking-face of a first pallet-stone of the pallet-lever during each locking phase. This phase is generated by the impulse pin of the balance which rests on a first horn of the fork, with the balance then moving the pallet-lever through an unlocking-angle. During the impulse phase, the second horn of the fork rests against the impulse pin and exerts a force on the latter as a result of the torque provided by the escape wheel, whose aforementioned tooth applies a force on an impulse-face of the first pallet-stone. During this impulse phase, the balance receives an impulse for maintaining its oscillations and the pallet-lever continues its rotational movement through an impulse-angle. Finally, during the safety phase, the impulse pin is released from the fork and describes another ascending and then descending supplementary arc. The pallet-lever then effects the end phase of its rotation through a safety-angle, called the “run to the banking”, which ensures that the impulse pin is released from the fork and an escape wheel tooth is correctly positioned on the locking-face of the second pallet-stone.
During the locking phase, the guard pin of the pallet-lever ensures that the pallet-lever remains substantially in its locking-angle position. In a main embodiment, there is provided a safety roller integral with the balance staff and having a slot to allow the pallet-lever to rotate during the coupling between the impulse pin and the fork. The end of the guard pin is located at a short distance from the lateral surface of the safety roller when the pallet-lever is in one of its two locking positions resting on a banking pin. In particular, during the locking phases, when an undesired force acts on the pallet-lever, the pallet-lever may leave its locking position on the pin concerned and the guard pin then comes into contact with the lateral surface of the safety roller, which will result in a disruption to the oscillating motion of the balance. This causes a problem for the proper operation of the regulator. In any event, it is desirable for such an event to be as brief as possible. In a mechanical movement where the source provides mechanical energy to the escape wheel to maintain oscillating motion, as is the case with the Swiss lever escapement described above, draw is caused in the locking phases or periods by the escape wheel cooperating with the pallet stone on which it rests. This mechanical draw defines a return force for the pallet-lever which alternately presses it against the banking pins and returns it to said banking pins after the timepiece movement is subjected to shocks or sharp accelerations which momentarily drive the pallet-lever in rotation.
There are also known timepiece movements wherein energy is provided directly to the balance by a drive source, the balance is then a mechanical drive element which, on the one hand, transmits energy to drive a gear train and on the other hand, is used for regulating the rate of the movement by means for counting its oscillations. In this regard, CH Patent 573136 discloses an electrically maintained balance and a system for counting its oscillations formed by an inverted lever escapement. Electromechanical movements of this type are arranged and operate very differently from timepiece movements equipped with a Swiss lever escapement. In particular, given that the escape wheel is not subjected to an energy source other than that provided by the balance, this escape wheel does not exert any draw as in the case of a mechanical movement in which the drive energy is provided by a barrel, which drives, in particular the escape wheel. Since there is no mechanical draw, this document indicates that draw can be obtained by a magnet mounted on the pallet-lever and two positioning pins a priori made of a ferromagnetic material. These pins continuously attract the pallet-lever, the overall force of attraction being directed towards the pin closest to the magnet, which enables the pallet-lever to be held in its locking positions in which it rests alternately on the pins.
The Swiss lever escapement has proved capable of good regulation of a mechanical timepiece movement. However, this type of escapement remains a complex mechanism and it is sensitive to shocks and to sharp accelerations as explained above. In particular, it is difficult to simultaneously optimise two important parameters which are the efficiency of such an escapement and safety in the event of shocks, since the pallet-lever is the first element affected by these two technical features. It is known that even a small vibration or any rebounding of the pallet-lever during the locking periods results in disruption to the balance (via the guard pin which comes into contact with the safety roller and rubs against the latter), which impairs efficiency and chronometry. In known embodiments, the draw of the pallet-lever and therefore the holding thereof against a banking pin limiting its rotation, are ensured only by a certain torque applied to the pallet-lever by the escape wheel. It is noted that this torque may be too low, or completely lacking during part of the supplementary arc of the pin/balance, particularly in optimised escapements with a substantially constant torque because the escape wheel in such mechanisms advances slowly.